WIP: point z-axis up, change some settings for OrbitControls

[mgeier]

I really like this project, but one thing I always found irritating was the behavior of the mouse controls for rotation.

There have been some discussions before, e.g.:

• panning, turntable rotation, z up: #28
• orbit control doesn't work: #89
• z up, orbit controls: https://github.com/maartenbreddels/ipyvolume/issues/156#issuecomment-407819104

I've tried to hopefully improve on that, but I guess some of it might not be the right way to do it. I'm hoping for plentiful feedback!

These are the changes I intended:

• point the z-axis up
• enable panning (in OrbitControls), and select the "good" panning mode

I might have done some unintended changes and it's quite likely that I've completely broken something.

I'm mostly interested in the "orbit" controls, I don't see why anyone would want the "trackball" controls (now that the "orbit" controls work much better).

I'm not sure what the initial camera position should be ...

[mgeier]

I've been mixing a few things here, I've made a separate PR that isolates the "enable panning" feature: #273.